The present invention relates to fluid controllers, and more particularly, to such controllers which are used to control the flow of fluid from a source to a fluid pressure actuated device, especially a device of the type having some sort of mechanical stop means to limit the movement or travel of the movable portion of the pressure actuated device. A typical example is in a hydrostatic power steering system in which the fluid controller controls and meters the flow of pressurized fluid to the vehicle steering cylinder.
The present invention is especially advantageous when used with a fluid controller of the type in which the valving comprises a pair of valve members, one of which is generally cylindrical and hollow and surrounds the other valve member. Such a controller is sold commercially by the assignee of the present invention under the trademark "ORBITROL".RTM. steering control unit.
One of the problems which has occurred in connection with the use of such controllers in hydrostatic power steering systems has been the occurrence of a "stiction" condition. Stiction occurs when the controller is operated to actuate the steering cylinder against the cylinder stops, after which the valve means is permitted to return to the neutral position. The above-described mode of operation results in trapping pressurized fluid in the cylinder, the conduit to the cylinder, and various ports and passages within the controller. In a typical controller of the type to which the present invention relates, this trapped, pressurized fluid surrounds a portion of the outer, hollow cylindrical valve member, and causes that portion of the valve member to be slightly deformed and "clamp" onto the inner valve member. The next time the operator attempts to initiate a steering maneuver, the hollow valve member being clamped on the other valve member makes it more difficult to rotate the steering wheel and initiate valving action. The resulting difficulty in turning the wheel is referred to as the "stiction" condition.
Those skilled in the art have attempted to overcome the problem of clamping and stiction in several different ways. One attempted solution has involved locating a number of axial grooves on the outer surface of the inner valve member, to reduce the clamping area, i.e., the area of possible clamping engagement between the inner and outer valve members.
Another solution attempted by those skilled in the art has been simply to increase the radial clearance between the inner and outer valve members, such that a greater amount of "collapse" of the outer valve member must occur in order to have any clamping engagement of the outer valve member on the inner valve member. Neither of the attempted solutions described above has been fully satisfactory in eliminating the stiction condition and maintaining basic steering function.